Digital v.s. Film Costs in Electron Microscopy

Preparation of a cross-sectional semiconductor IC device sample for SEM observation

By Prashanth Prasad, Gatan

SEM Characterization of semiconductor IC device packages is extremely essential for failure analysis and for evaluation of microelectronic package reliability. Observation in cross-section provides a wealth of information about the IC device such as layer thicknesses, layer structures, grain sizes of various crystals in the layers and the existence of voids and delaminations. Preparation of cross-sections involves three broad steps: cutting, mechanical polishing and etching. Etching can be performed using chemical reagents or an ion beam. Not all materials have known chemical etchants, and dealing with chemicals can be hazardous. As a result, there has been an increased interest in ion beam etching. This article discusses one such application of ion beam etching to the preparation of an IC device sample. Ion beam etching was accomplished using the Gatan Precision Etching and Coating System (PECSTM).

An illustration of the sample in cross-section, cut along the desired cutting plane, is shown in Fig 1.

Fig 1: Schematic illustration of sample in cross-section

Also indicated in Fig 1 are seven analytical areas of interest:

1. Boundary between the Copper lead and the silver spot, and between the silver spot and compound.
2. Boundaries between Copper lead, Silver spot, Silver epoxy and Compound.
3. Boundary between Passivation layer on the die and Compound.
4. Boundary between Silver epoxy and compound.
5. Boundary between the Gold ball and the Aluminum pad on die.
6. Boundary between the Copper lead and the silver spot, and between the silver spot and silver epoxy.
7. Boundary between the Die and silver epoxy.

Mechanical Preparation:
The bulk sample was cut along the desired cutting plane using a diamond saw. The cut sample was mechanically polished in the following order: 30 micron SiC paper, 15 micron SiC paper, 5 micron SiC paper, 1 micron Alumina lapping sheet, 0.25 micron Silica lapping sheet and 0.05 micron colloidal Silica suspension.

Ion beam Etching and Coating:
The sample was then etched in the Gatan PECS using these parameters:
1. 4.5 keV, 200 µA, 65^o fixed angle, 20 min. (High angle polishing removes surface damage.)
2. 4.5 keV, 200 µA, 10^o rock, 6 min. (Etching step at near-normal incidence)

The sample was immediately coated in the PECS with Pt using these parameters:
1. 6 keV, 200 µA, 30^o rock, 2.5nm Pt.

Results:

As seen in Fig. 2, a clear demarcation is achieved in Area 1 between the compound and Silver as well as between Silver and Copper. Irregularities at the boundary between Silver and Compound are revealed. No irregularities are seen at the boundary between Copper and Silver. The grain structure of both Copper and Silver is clearly visible as are the Porosity defects in Silver.

Fig 2: SEM micrograph of “Area 1”.

Good differentiation is achieved in Area 2 among the Compound, Copper and Silver layers. Delamination between Silver and Compound is seen at the side (Fig. 3) but not at the top (Fig. 4).

Fig 3: SEM micrograph of “Area 2” (side region) showing delamination between Copper and Compound

Fig 4: SEM micrograph of “Area 2” (top region) showing no delamination between Copper and Compound

In Area 3 (Fig. 5) and Area 4 (Fig. 6), we see a regular and continuous boundary between the passivation layer and the Compound and between the Silver epoxy and compound respectively. The grain structure of Silver in the epoxy is visible as well.

Fig 5: SEM micrograph of “Area 3”.

Fig 6: SEM micrograph of “Area 4”.

The grain structure of the Gold ball, conforming to the flow pattern, and the boundary between the Gold ball and the passivation layer are clearly observed in Fig 7. The presence of cavities at the fore mentioned boundary is noted.

Fig7:“SEM micrograph of “Area 5”.”

Area 6 and Area 7 revealed distinct, regular boundaries between Copper and Silver spot, Silver spot and Silver epoxy, and the Die and Silver epoxy.*

Conclusion:

Cross-sectional preparation of the IC device package was successfully achieved through etching with the PECS. Ion beam etching revealed discrete boundaries between the various layers in the sample as well as defects in some areas along the boundaries. Ion beam etching was also able to bring out the grain structures in all crystalline components of the sample. Thus, the sample preparation technique resulted in a high quality sample for SEM studies.

* For images of Area 6 and Area 7, or for more information about Areas 1-5, please contact the author at pprasad@gatan.com.

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